The weight of the air that makes up our atmosphere exerts a pressure on the surface
of the earth. This pressure is known as atmospheric pressure. Generally, the more
air above an area, the higher the atmospheric pressure, this, in turn, means that
atmospheric pressure changes with altitude.
For example, atmospheric pressure is greater at sea-level than on a mountaintop.
To compensate for this difference and facilitate comparison between locations with
different altitudes, atmospheric pressure is generally adjusted to the equivalent
sea-level pressure. This adjusted pressure is known as barometric pressure.
Barometric pressure also changes with local weather conditions, making barometric
pressure an extremely important and useful weather forecasting tool. High pressure
zones are generally associated with fair weather while low pressure zones are generally
associated with poor weather. For forecasting purposes, however, the absolute barometric
pressure value is generally less important than the change in barometric pressure.
In general, rising pressure indicates improving weather conditions while falling
pressure indicates deteriorating weather conditions